Current steering digital-to-analog converter (DAC) is widely used in modern analog and mixed-signal circuit system especially for high sampling rate applications. It consists of an array of current sources, the number of array depends on DAC's decoding scheme either thermometer, binary or segmented. The DAC linearity is mainly determined by the matching of the array of current sources.
When designing a high speed DAC many tradeoffs have to be taken into account. These include the type of the DAC (current steering or a voltage output DAC), the number of bits and required speed. For high speed DACs usually a current steering architecture is selected because of the speed advantages of this converter type with respect to the voltage output DAC. In most current steering DAC designs a combination of binary and thermometer code weighting is used. The thermometer code is used for the Most Significant Bits (MSBs), while binary code is used for the Least Significant Bits (LSBs). A combination of thermometer and binary codes is called segmentation: 0% segmentation means a fully binary converter and 100% segmentation means a full thermometer code converter. The DAC designer has to choose the optimum amount of segmentation and take into account the physical problems associated with the segmentation. Current steering DACs with a low amount of segmentation have the advantage of simplicity. They only need a few current sources and switches. The disadvantage of low segmentation is the possible larger DNL compared to the converter converters with larger segmentation. Converters with larger segmentation result in a lower DNL, but its larger area, more complex decoding scheme, and the difficulty to match the timing of different current sources bring other issues.
The basic principle of current steering DACs is the summation of currents according to the input. In a binary current steering converter, the current sources are connected parallel to each other. These current sources are connected to switches; the switches connect the current source to the output node. The switches are controlled by the input code of the DAC. The output current of the DAC is therefore proportional to the input code word. The output node of the DAC is connected to a resistor. This resistor converts the output current of the DAC into a voltage.